The present invention generally relates to display panels, and, more particularly, to a display controller for driving a display panel.
Display panels are widely used in devices including watches, gaming consoles, computers, mobile phones, televisions, cameras, and in automobiles for displaying information, often using images. Examples of display panels include a liquid crystal display (LCD) panels, light emitting diode (LED) display panels, and plasma display panels.
A display panel is driven by an integrated circuit (IC) that includes a memory, a processor, a display controller, and a clock generator. The memory stores images to be displayed on the display panel in the form of graphic data layers. The graphic data layers, some of which may overlap, include pixel data that is blended and displayed on the display panel.
The processor provides access to the display controller for fetching the pixel data from the memory over a system bus. The display controller blends the pixel data and provides the blended pixel data to the display panel based on a clock signal (i.e., a “reference clock signal”) generated by the clock generator. Spectral components of the reference clock signal contribute to electromagnetic interference (EMI) emissions or radiation. The EMI emissions may cause undesirable interference with other components of the display controller, the display panel, and other nearby circuits or electronic equipment. Generating the reference clock signal at a fixed frequency concentrates the energy of the radiation in a narrow spike having a large amplitude. The amplitude of the spike usually exceeds the EMI limit set by government agencies such as the Federal Communication Commission (FCC), which strictly regulates the amount of radiation or EMI emissions that an electronic device can generate. Further, in case of high resolution display panels, the size of the blended pixel data is large and requires higher clock rates, which leads to an increase in the intensity of the radiation and further contributes to increasing the amplitude of the spike.
One way to reduce EMI emissions is to vary or modulate the frequency of the reference clock signal, thereby generating a modulated clock signal, and provide the blended pixel data to the display panel using the modulated clock signal. In this case, the display controller includes a clock divider that modulates the frequency of the reference clock signal over a frequency range (also known as “frequency spectrum”) based on a clock dividing ratio received from the processor. The processor varies the clock dividing ratio periodically over a range for modulating the frequency of the reference clock signal. As the frequency of the reference clock signal is varied periodically with time, the intensity of the radiation at a particular frequency is reduced, thereby reducing the amplitude of the spike at the particular frequency. This technique is known as spread spectrum. The maximum frequency range over which the frequency of the reference clock signal can be modulated is limited by a tolerance limit of the reference clock signal.
The display controller includes data buffers that buffer the blended pixel data and synchronize the transfer of the blended pixel data to a frequency of the modulated clock signal. When the number of graphic data layers containing pixel data is large, the size of the blended pixel data to be transferred to the display panel is large and hence, more pixel data must be fetched from the memory. If the frequency of the modulated clock signal is at a higher end of the frequency spectrum, the rate at which the blended pixel data is transferred to the display panel exceeds the rate at which the pixel data are fetched from the memory, which leads to under-run of the data buffers, resulting in visual artifacts.
Thus, it would be advantageous to have a display controller that modulates the reference clock signal based on the amount of pixel data being fetched from memory, thereby reducing data buffer under-runs.